The transonic range may be defined as the range of air speed in which both subsonic and supersonic airflow conditions exist around a body. It is largely dependent on the body shape, curvature and thickness-chord ratio, and can be broadly taken as Mach 0.8-1.4.
For simplicity, in this specification the terms “transonic fan” and “transonic fan blade” will be used to refer to a fan and a fan blade intended to operate substantially in the transonic range.
A significant proportion of the aerodynamic inefficiency of a transonic fan is due to the loss associated with the shock wave forming near the tip of the blade. A known way to reduce this loss is to design the suction surface of the blade, upstream of the shock wave position, with near-zero curvature. This minimises the expansion of the flow and thereby minimises the pre-shock Mach number.
In a conventional transonic fan, the covered passage formed by two adjacent blades first converges, before diverging further downstream. That is to say, the cross-sectional area of the first (upstream) part of the passage reduces, and the cross-sectional area of the later part of the passage increases.
However, the low curvature of the suction surface results in the flow area (the area of the passage normal to the flow) varying slowly in the vicinity of the shock wave, thereby causing the position of the shock to be very sensitive to small geometric imperfections in adjacent blades. The change in shock position causes a significant change in the untwist of the blades (the total deflection generated by the centrifugal and aerodynamic loads), which in turn further changes the shock position. If the aerodynamic loads are sufficiently high and the structure sufficiently flexible, this feedback mechanism results in the nominal untwist deflections becoming unstable with respect to geometric variability.
Because the shock wave cannot sit in a converging passage, it must either sit ahead of the covered passage, or must “jump” into the diverging part of the passage. This large and sudden change in the shock position causes a correspondingly large change in the untwist of the blades, which in turn further changes the shock position, thus leading to instability.
It is therefore an object of the invention to provide a transonic fan blade in which the untwist behaviour is more stable with respect to small geometric imperfections.